The invention is in the area of drilling fluids used in connection with drilling wells, such as oil wells and water wells. The invention is also directed towards a drilling apparatus and process.
In drilling a well or other similar borehole, a drill bit is operatively coupled, usually by a drill string, to a drive which rotates the drill bit to cause the drill bit to bore into the earth. A drilling fluid, or drilling mud, is circulated through the borehole annulus. The drilling fluid passes through the drilling string and to the surface through the drill bit for cooling and lubricating the drill bit and for carrying rock cuttings generated by the cutting action of the bit to the surface. The drilling fluid may be a gas, but more typically is a liquid.
Many liquids suitable for use as drilling fluids are known. Some drilling fluids are oil-based. Such oil-based fluids suffer from a number of drawbacks, particularly, their adverse effects on the environment. Oil-based fluids also can be costly to purchase and to dispose of.
Other drilling fluids are water-based or constitute an emulsion of oil in water or water in oil. Such fluids often are inexpensive compared to oil-based fluids, and are less costly to dispose of. Water-based fluids also pose less of a risk to the environment than do oil-based fluids. One drawback associated with water-based fluids is that the water in the fluid tends to promote borehole instability, particularly when shale is encountered in the drilling process. The water-based fluid may adsorb and absorb into pores in the shale, thus causing the shale to swell and thereby tending to cause the borehole to collapse.
In recognition of this drawback, water-based drilling fluids have incorporated an alkyl glycoside, such as methyl glucoside, into the drilling fluid. It is known that methyl glucoside serves as a borehole stabilizing agent that functions by inhibiting shale swelling. The mechanism of action of methyl glucoside is not fully understood, but is believed to be associated with gel formation or an ion exchange phenomenon. It is generally believed that the methyl glucoside, in adding gel strength to the drilling fluid, causes more rapid formation of filter cake on the borehole wall thereby allowing less water filtrate to reach the surrounding shales. This is believed to reduce swelling and sloughing of the shale.
While known drilling fluids that include methyl glucoside are satisfactory, there remains room for improvement in such fluids in the area of borehole stability. It is a general object of the invention to provide a drilling fluid that is suitable for use in borehole drilling. Another general object is to provide a drilling apparatus and process that incorporate the drilling fluid of the invention.
It has now been discovered that both maltodextrins, on the one hand, and hemicellulose-containing materials, on the other hand, function to assist glycosides in promoting borehole stability in a drilling fluid. It has further been found that carboxyalkyl starches, in particular carboxymethyl starches, also function to assist glycosides in promoting borehole stability. Surprisingly, these ingredients promote borehole stability more so than other organic species of similar origin or chemical structure when used in conjunction with an alkyl glucoside. In accordance with the invention, a drilling fluid comprises a liquid base, an alkyl glycoside, which preferably is methyl glucoside; and a borehole stability agent that comprises a maltodextrin, a carboxyalkyl starch, and/or hemicellulose. Also encompassed by the invention is a drilling fluid that includes methyl glucoside and cellulose. The invention also encompasses a drilling apparatus and a process for drilling. The drilling apparatus comprises a drilling string that is fluidically coupled to a source of drilling fluid, the drilling fluid including in one embodiment a liquid base, an alkyl glycoside, and one or more of a maltodextrin, a carboxyalkyl starch, and a hemicellulose-containing material and, in another embodiment, a liquid base, an alkyl glycoside, and cellulose. The process of the invention includes the steps of circulating the drilling fluid of the invention through a drill string as a borehole is cut into the earth, optionally in conjunction with hemicellulose, a maltodextrin, a carboxyalkyl starch and/or cellulose.
It has further been found that desugared molasses functions effectively as a borehole stabilizing agent. In accordance with another embodiment of the invention, a drilling fluid comprises a liquid base, optionally an alkyl glucoside, and a borehole stabilizing agent that comprises desugared molasses solids. Also encompassed by this embodiment of the invention are a drilling apparatus and a process for drilling. More generally, the various borehole stabilizing agents discussed hereinabove may be combined with the alkyl glucoside to form a drilling fluid. Thus, for instance, the drilling fluid may comprise an alkyl glucoside, desugared molasses solids, and one or more of hemicellulose, a maltodextrin, and a carboxyalkyl starch.
Other features and embodiments of the invention are discussed hereinbelow and are set forth in the pending claims.